2008 335i changed valve cover gasket and plugs with battery disconnected for several days, after repairs made car ran poor, shifted hard, steering hard, idle erratic tripping reduced power and SES light ... after 2 weeks driving steering and shifting better but reduced power and SES still come on so no turbo power, car was fine before disconnecting battery. both DIY's for valve cover gasket replace indicated the battery disconnect with no warning about computer control module losing settings, miss the performance, no fun without turbo power, please help

Hi Johnny

Have you read the codes stored and reset them also your car may think it has a new battery and I know certain models need the battery registering. My car is to old and is happy to have a battery at all!! I have read that an unregistered battery can have all s sorts of odd effects from no start to no charging, I  would start with a code read and reset then register the battery.

Dave

thanks for your reply Greydog, the battery was registered and never been an issue. the issue seems to be the computer failing to get the idle speed right, even after driving it over a hundred miles. what is odd is that if i gun it on starting the turbos do kick in and no warning light comes on but after maybe ten seconds it does come on every time. which indicates the turbos are good. i dont have a scan tool to read the codes but am looking into purchasing one, tho there are so many its hard to know which to buy, and not knowing if a scan tool can set the idle what sense is there in buying one till i know more. the car idled very rough at first three weeks back after reconnecting the battery but its much smoother now, and idles better than ever once the reduced power warning comes on, it always comes on with a slight surge in rpm. car runs great but for no turbo power due to limp mode and SES light stays on all the time. these bmw's are frustrating creatures to be sure, but great fun when right.

Morning Johnny 

Didn't you know the initials BMW stand for Break My Wallet 

Buying a reader is not just a good step, it's essential. There are many generic readers which will give generic codes but they will not be able to read specific modules on the BMW bus system so tracking faults is harder. So will need a reader with software that is BMW specific or enabled. Foxpro or Autologic but these are professional standard and carry the price tag.

If you have a laptop I would look at BMW 1.4.0 (take a look on line) it is about £15 and once loaded you can read individual control modules (there are 3 pages though not all are applicable to your model) The other I would consider is INPA very comprehensive though I found it a pain to load plus the diagnostics are in German so a bit of translation could be required. That said it is very good I just prefer 1.4.0 as it gives the same information but in English. Both may program's are 10+ years old so current versions are quite probably more user friendly.

Where are you in the UK you may be near a member who has a Code reader? I am in Sussex

Dave